gdb/python: Reuse gdb.RegisterDescriptor objects where possible

gdb/python: Reuse gdb.RegisterDescriptor objects where possible

Instead of having the gdb.RegisterDescriptorIterator creating new
gdb.RegisterDescriptor objects for each regnum, instead cache
gdb.RegisterDescriptor objects on the gdbarch object and reuse these.

This means that for every gdbarch/regnum pair there is a single unique
gdb.RegisterDescriptor, this feels like a neater implementation than
the existing one.

It is possible for a user to see (in Python code) that the descriptors
are now identical, but as the descriptors are read-only this should
make no real difference.

There should be no other user visible changes.

gdb/ChangeLog:

	* python/py-registers.c (gdbpy_register_object_data): New static
	global.
	(gdbpy_register_object_data_init): New function.
	(gdbpy_new_register_descriptor): Renamed to...
	(gdbpy_get_register_descriptor): ...this, and update to reuse
	existing register descriptors where possible.
	(gdbpy_register_descriptor_iter_next): Update.
	(gdbpy_initialize_registers): Register new gdbarch data.

gdb/testsuite/ChangeLog:

	* gdb.python/py-arch-reg-names.exp: Additional tests.